This invention relates to colour video projectors of a type employing red, green and blue cathode ray tubes (CRTs), dichroic mirrors and a final single lens through which the red, green and blue light from the CRTs is projected onto a screen. Projectors of this type commonly are used as monitors for computer graphics and video applications. Additional details of such a projector may be found in U.S. Pat. No. 4,607,280 issued Aug. 19, 1986, Maarjus Kurg entitled "Video Projector" and assigned to the assignee of the present application, the disclosure of which is incorporated herein by reference.
Projectors of the type noted above have a substantial advantage over three lens systems, in which the red light, green light and blue light pass separately through three separate final lenses, in that the latter require a tedious and time-consuming operation to be performed to achieve convergence on the screen. The former, on the other hand, have the advantage of portability because each CRT is converged inside the projector at the factory, enabling easy customer set up at any location.
Electrohome Limited of Kitchener, Ontario, Canada, just has begun to sell a colour video projector known as the ECP1000. In the evolution of the design of this projector it was found at one stage to have two disadvantages, the first being non-uniform colour temperature across the width of the screen, and the second being a marked decrease in brightness as high as 75% from the centre to each edge of the screen.
The instant invention is directed in its primary aspect to a solution to the first problem. As a secondary aspect a solution to the latter problem also is provided.